The invention described herein relates to oscillators for generating electrical oscillation waves or clock pulses, and more particularly to ring oscillators. Ring oscillators are characterized by having an odd number of active elements, such as amplifiers or inverters connected in series or cascade in a ring configuration. In a ring oscillator the output of one active element is connected to the input of another active element to form a closed chain or loop. The active elements of a ring oscillator are connected and biased such that they generate self-sustained oscillations. The states may be conducting and non-conducting states thereby generating self-sustained oscillations.
Oscillators are vital components in a variety of electronic and microelectronic circuits, including analog circuits, digital circuits, mixed analog-digital circuits and radio frequency circuits in sensors, radio receivers, radio transmitters, navigation equipment, etc. Within these devices oscillators are used or in conjunction with as Phase Locked Loops (PLL), clock generators, frequency generators, frequency multipliers, frequency dividers, and mixers among other circuit components.
One type of oscillator is a ring oscillator. Connecting an odd number of inverting gain stages in a ring, as shown generally in FIG. 1 can make a ring oscillator. FIG. 1 illustrates three inverting amplifiers in series in a closed ring configuration. Each individual inverting amplifier may be implemented as a CMOS inverter, as shown in FIG. 2.
In actual circuit implementation, ring oscillators are subject to jitter and noise. One source of noise is oscillator phase noise, while a frequent source of jitter is random phase delays that are converted to random time delays along the inverter ring. The noise and jitter is undesirable, especially in an oscillator incorporated in a low voltage integrated circuit having a relatively low input and/or output voltage. Therefore, there exists a need for improved oscillators that are less subject to jitter and noise, have a wide tuning range, and a fast switching time. As will be seen, the invention provides such an improved oscillator in an elegant manner.